Phenylketonuria: Direct and indirect effects of phenylalanine

Abstract High phenylalanineconcentrations in the brain due to dysfunctional phenylalanine hydroxylase(Pah) are considered to account for mental retardation in phenylketonuria(PKU). In this study, we treated hippocampal cultures with the amino acid in order to determine the role of elevated levels of phenylalaninein PKU-related mental retardation. Synapse density and dendritic length were dramatically reduced in hippocampal cultures treated with phenylalanine. Changes in cofilinexpression and phosphorylation status, which were restored by NMDA, as well as reduced activation of the small GTPase Rac1, likely underlie these structural alterations. In the Pah enu2 mouse, which carries a mutated Pah gene, we previously found higher synaptic density due to delayed synaptic pruningin response to insufficient microgliafunction. Microgliaactivity and C3 complement expression, both of which were reduced in the Pah enu2 mouse, however, were unaffected in hippocampal cultures treated with phenylalanine. The lack of a direct effect of phenylalanineon microgliais the key to the opposite effects regarding synapse stability in vitro and in the Pah enu2 mouse. Judging from our data, it appears that another player is required for the inactivation of microgliain the Pah enu2 mouse, rather than high concentrations of phenylalaninealone. Altogether, the data underscore the necessity of a lifelong phenylalanine-restricted diet.